This invention relates to mass air flow sensors for measuring the mass flow of air or other fluid. The invention relates also to methods of making such sensors, including batch fabrication methods.
Mass air flow sensors employing the principle of hot film anemometry are well known for use in instrumentation applications, including especially various process control applications. Typically, electrical current is used to maintain a resistor film at a certain temperature above the temperature of the air flow by electronic sensing and feedback circuits. The electronic circuit senses changes in heat transfer rate to the air flow. Such sensors are used for measuring the mass of a fluid flow, for example, the mass of air flowing in an air conditioning system, the air intake of a motor vehicle engine, gas chromatography eluted gas flows, etc. One such mass air flow sensor (MAFS) is that shown in U.S. Pat. No. 4,594,889 to McCarthy, the disclosure of which is incorporated herein by reference. The McCarthy device is a so-called monolithic silicon mass air flow sensor (SiMAFS), preferably batch fabricated by chemically etching ("micromachining") 100-millimeter (4-inch), oriented single-crystal silicon wafers. The silicon wafer is typically about 600 .mu.m thick, the top and bottom surfaces being (100) crystallographic planes. The McCarthy patent discloses MAFS having an air flow opening formed through a silicon substrate. Elongate silicon members bridge across the air flow opening and carry platinum metal coatings exposed to the air flow for use as the sensor's hot wire and temperature sensing wire. Conductive terminals at the ends of the silicon members are in electrical contact with the metal coatings. An integrated circuit associated with the silicon chip generates a signal corresponding to the mass air flow. The MAFS of the McCarthy patent is especially suitable for use in measuring the mass of air introduced into an automobile engine. For such application, the MAFS would be located typically between the air cleaner and the engine throttle body.
A recognized difficulty concerning MAFS of the type disclosed in the McCarthy patent is the excellent thermal linkage between the main silicon chip and the silicon members unitary therewith which bridge across the air flow opening carrying the metal resistor films. This thermal linkage imposes a constraint on the size of the silicon members. For silicon members larger than about 50 .mu.m across, the heat loss to the chip via conduction becomes significantly large. This reduces the sensitivity of the sensor and requires greater power levels for operation of the sensor. Temperature compensation is also made more difficult. The influence of changes in chip temperature on the accuracy of measurement is increased. While these difficulties can be reduced by making the silicon members smaller in cross-sectional area, they then become more fragile, perhaps even breaking under the impact of dust particles in air flows of sufficient velocity. In the automotive engine application mentioned above, for example, the sensor preferably can survive and operate under conditions of vibration, temperature extremes, and bombardment by dust and the like for years without requiring service. Thus, for such prior known MAFS, thermal isolation, and hence accuracy, may be compromised with robustness of the sensor.